Stopping distance
📖Definition
The stopping distance of a vehicle is the total distance from when the driver first sees the hazard to when the vehicle finally stops:
stopping distance = thinking distance + braking distance
Thinking distance
Thinking distance is the distance travelled by the car during the driver’s reaction time — between seeing the hazard and pressing the brake pedal.
thinking distance = speed × reaction time
For an average driver, reaction time is ~0.6 s. At 30 mph (~13 m/s) that is ~8 m.
Factors that increase thinking distance:
- Higher speed (proportional).
- Tiredness, alcohol, drugs (including some prescriptions), or distractions (phone, food). All increase reaction time.
Braking distance
Braking distance is the distance the car travels after the brakes are applied until it stops.
½ × m × v² = F × d (work done by brakes = kinetic energy lost)
So at constant braking force, braking distance is proportional to v² — doubling the speed quadruples the braking distance.
Factors that increase braking distance:
- Higher speed (proportional to v²).
- Worn brakes or worn tyres — less friction.
- Wet, icy or oily road — less grip.
- Greater mass of the vehicle (more KE for the same brakes to dissipate).
- Downhill slope — gravity adds to KE during braking.
Energy view
When a car brakes, kinetic energy of the car is transferred mainly to thermal energy in the brake discs and pads (and a little to sound). Brakes get hot. If the brakes are repeatedly applied (e.g. long downhill drive) they can overheat → brake fade.
Reaction time experiment (ruler-drop)
- Partner holds a ruler vertically; you place thumb and forefinger either side of the 0 cm mark, not touching.
- They release without warning; you grab the ruler as quickly as you can.
- Record the distance the ruler fell (in m).
- Use t = √(2s/g) with g = 9.8 m/s² to convert distance to reaction time.
- Repeat several times and take a mean to reduce random error.
A typical reaction time is ~0.20 s. Tired or distracted subjects give consistently larger values.
Edexcel exam tip
When discussing factors that affect stopping distance, always specify whether you are talking about thinking or braking distance — alcohol affects thinking distance only; rain affects braking distance only. Mixing them up loses easy marks.
AI-generated · claude-opus-4-7 · v3-edexcel-combined-science-leaves